Conception Date Calculator

Two ways to date conception, and why they can disagree

There are two routes to an estimated conception date, and both start from something a person usually knows. The first works back from the due date: subtract 266 days. The second works forward from the first day of the last period: add the cycle length minus 14, which is the day before ovulation on a typical cycle.

The 266 comes from the arithmetic of pregnancy dating. A due date is 280 days from the first day of the last period, and ovulation sits about 14 days into a 28 day cycle, so 280 minus 14 is 266. On a 28 day cycle the two routes give the same answer by construction. On a 35 day cycle they give the same answer only if the due date was calculated with that 35 day cycle, which is what the calculator on this page does.

They disagree when the due date came from somewhere else. An early ultrasound, an IVF transfer date or a provider's own estimate can set the due date without reference to the period, and then the two routes split by the size of that correction. A due date moved a week later by ultrasound moves the conception estimate a week later too, while the period route stays where it was.

Worked example: a January 5, 2026 last period

Take a last period that started January 5, 2026 and a 28 day cycle.

• Ovulation and conception: January 5 plus 14 days is January 19, 2026
• Due date: January 5 plus 280 days is October 12, 2026
• Reverse check: October 12 minus 266 days is January 19, 2026

The two routes agree to the day, which is the expected result when the cycle is 28 days and the due date came from that same period. Now move the cycle to 35 days. Ovulation moves to day 21, so conception becomes January 26, 2026, and the due date moves with it to October 19, 2026. Subtract 266 from that new due date and the answer is January 26 again.

Gestational age is the part that surprises people. On January 19 the pregnancy is already dated at 2 weeks 0 days, because the count starts on the first day of the last period, about two weeks before there was anything to conceive. A test taken two weeks later, on February 2, 2026, is read as a 4 week 0 day pregnancy, because that is 28 days from the period date and the hormone shows up a few days after implantation.

Why the answer is a 5 day window, not a date

The calculator returns one date, and the honest reading of that date is a range. Sperm survive in the reproductive tract for up to 5 days, and the egg is viable for roughly 12 to 24 hours after release. Conception can therefore happen on the day of ovulation or on any of the four days before it, which makes the fertile window 5 days wide.

For the January 5 example that window is January 15 to January 19, 2026. If ovulation came a day late, conception lands on January 20 and the window shifts with it. This is why a conception date calculated to the day is a convention rather than a measurement: nobody can tell whether a given pregnancy started on the 17th or the 19th without tracking ovulation directly.

Implantation adds a second delay. The fertilized egg travels down the fallopian tube and implants in the uterine lining 6 to 12 days after ovulation, which for this example is January 25 to January 31, 2026. That is the point at which the body starts making the hormone a pregnancy test detects, which is why a test taken a week after conception is usually still negative.

Cycle length moves the date by a week or more

Ovulation is not fixed at day 14. It sits about 14 days before the next period, so a longer cycle means later ovulation, not a longer second half. A 35 day cycle ovulates around day 21; a 24 day cycle ovulates around day 10. The table applies that rule to the same January 5, 2026 period.

Eleven days separate the earliest and latest conception date in that table, which is a wider spread than most people expect from a number that gets quoted to the day. Irregular cycles widen it further, because the length can change from month to month. If cycles run anywhere from 26 to 34 days, the conception estimate is a band of about 8 days before the other uncertainties are counted.

What a due date is, and how far it can move

A due date is an estimate built on a 280 day count from the last period. Only about 4 in 100 babies arrive on the date itself, and the spread around it is wide: two weeks early and one week late are both ordinary. The date is a planning tool for scheduling appointments and leave, not a prediction.

Ultrasound dating is the usual correction. A first trimester scan measures the embryo and dates the pregnancy to within about 5 to 7 days, which is more accurate than a remembered period date, especially for anyone with irregular cycles. The ACOG committee opinion on estimating the due date sets out when a scan should override the period: a difference of more than a week early in pregnancy is enough to redate.

Once a scan has set the due date, that date is the one to work from. Subtracting 266 days from it gives the conception estimate used by the calculator, and it will not match the period route if the scan moved things. That mismatch is information, not an error.

The two weeks that get counted before conception

Pregnancy is dated from the last period, not from conception, which is why a positive test at what feels like two weeks after conception is called a four week pregnancy. The first two weeks of the 40 week count happen before sperm and egg meet, and the convention exists because the period date is something most people can report while the conception date is not.

That convention runs through everything downstream. The end of the first trimester is 13 weeks 0 days from the last period, which is 11 weeks from conception. Viability discussions at 24 weeks are 22 weeks from conception. When a provider says 30 weeks, the embryo has been developing for 28. The offset is always the same 14 days on a 28 day cycle, and it grows to 21 days on a 35 day cycle.

This is the reason the calculator reports gestational age at conception as 2 weeks 0 days rather than day zero. Anyone comparing a conception date to a scan report needs to hold that offset in mind, or the numbers look two weeks apart when they agree.

IVF, a known ovulation day, and dates that beat the estimate

Some pregnancies come with a date that is better than any estimate. IVF is the clearest case, because the transfer date is on a chart and the embryo's age is known exactly.

For a day 5 transfer the 261 day rule is the 266 day rule with the 5 days of embryo culture subtracted, and the conception date is the transfer date minus those same 5 days. The NICHD pregnancy pages describe the underlying biology if you want the detail behind the arithmetic.

Everything else is an estimate. A dating scan, a period date and a remembered ovulation all carry a few days of uncertainty, and the 5 day window on this page is the floor, not the ceiling.

What this date is good for, and what it is not

A conception date is useful for a handful of practical things: understanding a scan report, working out which cycle a pregnancy started in, answering a question from a relative, or filling in a baby book. It is also the number people reach for when they are trying to work out paternity, and that is where it stops being useful. A date with a 5 day window cannot settle a question that needs a DNA test.

The March of Dimes pregnancy resources put the uncertainty plainly: the estimated date is a best guess, and providers adjust it when better information arrives. Nothing on this page changes what your provider has written in your chart, and the chart is what matters for care.

To use the calculator, enter whichever date you have. A due date alone is enough: the tool subtracts 266 days. A last period plus the cycle length is enough: the tool adds the cycle length minus 14. Enter both and the last row shows how far apart the two answers are, which is a quick way to see whether the due date was built from that period or from something else.

This is a reference estimate, not medical advice, and it cannot date a pregnancy to the day. Confirm anything that matters with your own provider. All calculations run in your browser, and nothing you enter is uploaded or stored.